Computed tomography is an imaging procedure that has been widely used in the medical field. In a procedure for computed tomography, an x-ray source and a detector apparatus are positioned on opposite sides of a portion of a patient under examination. The x-ray source generates and directs a x-ray beam towards the patient, while the detector apparatus measures the x-ray absorption at a plurality of transmission paths defined by the x-ray beam during the process. The detector apparatus produces a voltage proportional to the intensity of incident x-rays, and the voltage is read and digitized for subsequent processing in a computer. By taking a plurality of readings from multiple angles around the patient, relatively massive amounts of data are thus accumulated. The accumulated data are then analyzed and processed for reconstruction of a matrix (visual or otherwise), which constitutes a depiction of a density function of a volume of the bodily region being examined. By considering one or more sections in the volume, a skilled diagnostician can often diagnose various bodily ailments such as tumors, blood clots, etc.
Computed tomography has found its principal application to examination of bodily structures or the like which are in a relatively stationary condition. However, currently available computed tomographic apparatus may not be able to generate tomographic images with sufficient quality or accuracy due to physiological movement of a patient. For example, beating of a human heart and breathing have been known to cause degradation of quality in CT images. U.S. Pat. No. 3,952,201 issued to Hounsfield, describes a system that compensates for a blurring of a radiograph due to heart motions by collecting image data of a patient at different angles while monitoring the patient's heart. The image data and the heart motion data are then correlated in a computer, and image data that was obtained when the heart's motion exceeded a threshold level is rejected.
Degradation of quality of CT images due to patient's breathing is more difficult to address than that associated with heart motion. Patients' breathing poses a unique problem to CT imaging that is different from heart motion. This is because the pattern and the period of a patient's breathing cycle is generally less consistent when compared to those of the patient's cardiac cycle. As such, while a particular phase of a cardiac cycle may be determined with sufficient accuracy, a particular phase of a breathing cycle may not be as easily determined.
Existing CT imaging systems may use a camera to determine patient position. In such CT imaging system, a marker block having a plurality of reflective elements is placed on a patient's chest. The camera is then used to sense the reflective elements on the marker block. By determining the positions of the reflective elements, a position and/or orientation of the marker block can be determined. The positional data is then considered in the construction of CT images so that patient's movement is addressed. However, adding a camera to an existing CT imaging system would add cost to the system. Also, in some cases, it may be difficult or impossible to install a camera in an operation room due to various mechanical and electrical constraints.